我按照http://cran.r-project.org/bin/linux/ubuntu/README中给出的指令使用镜像http://ftp.iitm.ac.in/cran/
sources.list文件现在看起来像：
# deb cdrom:[Ubuntu 14.04 LTS _Trusty Tahr_ - Release amd64 (20140417)]/ trusty main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://sa.archive.ubuntu.com/ubuntu/ trusty main restricted
deb-src http://sa.archive.ubuntu.com/ubuntu/ trusty main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://sa.archive.ubuntu.com/ubuntu/ trusty-updates main restricted
deb-src http://sa.archive.ubuntu.com/ubuntu/ trusty-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://sa.archive.ubuntu.com/ubuntu/ trusty universe
deb-src http://sa.archive.ubuntu.com/ubuntu/ trusty universe
deb http://sa.archive.ubuntu.com/ubuntu/ trusty-updates universe
deb-src http://sa.archive.ubuntu.com/ubuntu/ trusty-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://sa.archive.ubuntu.com/ubuntu/ trusty multiverse
deb-src http://sa.archive.ubuntu.com/ubuntu/ trusty multiverse
deb http://sa.archive.ubuntu.com/ubuntu/ trusty-updates multiverse
deb-src http://sa.archive.ubuntu.com/ubuntu/ trusty-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://sa.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src http://sa.archive.ubuntu.com/ubuntu/ trusty-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu trusty-security main restricted
deb-src http://security.ubuntu.com/ubuntu trusty-security main restricted
deb http://security.ubuntu.com/ubuntu trusty-security universe
deb-src http://security.ubuntu.com/ubuntu trusty-security universe
deb http://security.ubuntu.com/ubuntu trusty-security multiverse
deb-src http://security.ubuntu.com/ubuntu trusty-security multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
#deb http://archive.canonical.com/ubuntu trusty partner
#deb-src http://archive.canonical.com/ubuntu trusty partner
## This software is not part of Ubuntu, but is offered by third-party
## developers who want to ship their latest software.
deb http://extras.ubuntu.com/ubuntu trusty main
deb http://archive.canonical.com/ trusty partner
#deb-src http://archive.canonical.com/ trusty partner
deb-src http://extras.ubuntu.com/ubuntu trusty main
deb http://ftp.iitm.ac.in/cran/bin/linux/ubuntu trusty/
运行sudo apt-get update后，我收到了几条错误信息，其中一条我相信是相关的：
Err http://ftp.iitm.ac.in trusty/ Packages
sources.list文件有问题，不允许恢复所有存档 . 我是ubuntu的新手 . 我该怎么办？